1. Field of the Invention:
This invention relates to an automatic transmission equipped with an exhaust brake, in which the actuating pressure of a brake incorporated in a planetary gear mechanism of the automatic transmission is varied in dependence upon vehicle velocity to lighten the load acting upon the brake.
2. Description of the Prior Art:
A vehicle equipped with an exhaust brake apparatus and an automatic transmission is disclosed e.g. in the specification of Japanese Patent Application Laid-Open (Kokai) No. 59-80561.
Automatic transmissions incorporate a planetary gear mechanism, of which various types have been proposed. One such planetary gear mechanism is illustrated in FIG. 1.
With reference to FIG. 1, a torque converter 10 has a well-known structure which includes a pump 12, a turbine 13 and a stator 15. Also included is a high-gear clutch LC. The pump 12 is directly coupled to a crankshaft 17 on the engine side, and the turbine 13 is connected to a turbine shaft 22, which serves as the input shaft. The turbine shaft 22 is directly coupled to a carrier 23 constituting an overdrive mechanism 11. Rotatably supported on the carrier 23 is a planetary pinion 24 that meshes with a sun gear 25 and ring gear 26. A multiple-disc clutch Co and a one-way clutch Fo are arranged in series between the sun gear 25 and carrier 23. A multiple-disc brake Bo is provided between the sun gear 25 and a case 27 for the overdrive mechanism.
The sun gear 26 of the overdrive mechanism 11 is coupled to the input shaft 31 of a planetary gear mechanism 35. A multiple-disc clutch C1 for forward drive is provided between the input shaft 31 and an intermediate shaft 32, and a multiple-disk clutch C2 for reverse drive is arranged between the input shaft 31 and a sun gear shaft 33. Provided between the sun gear shaft 33 and the case 35 of the planetary gear mechanism are multiple-disc brakes B1, B2 and a one-way clutch F1. The sun gear 22 is provided with sun gears 38, 39 that mesh respectively with planetary pinions 36, 19 constituting two single planetary pinions. One of the planetary pinions, namely pinion 19, meshes with a ring gear 43, which is coupled to the intermediate shaft 32. The other planetary pinion, namely pinion 36, meshes with a ring gear 40 coupled to an output shaft 20. The ring gear 40 is coupled to planetary pinion 19 via a carrier 41. A multiple-disc brake B3 and a one-way clutch F2 are arranged in parallel between a carrier 37 of the other planetary pinion 19 and a case 34.
In the gear mechanism so arranged, a variety of transmission ranges can be obtained by engaging and disengaging the various clutches and brakes in dependence upon throttle opening, vehicle velocity and traveling conditions by means of a hydraulic control circuit. Table 1 illustrates the relationship between each range and the engaged/disengaged states of the brake and clutches. In Table 1, O indicates the engaged state and X the disengaged state. Further, indicates a freewheeling state during engine braking.
TABLE 1 ______________________________________ One-Way Operating Element Clutch Range C0 C1 C2 B0 B1 B2 B3 F1 F2 ______________________________________ Parking (P) O X X X X X X Reverse (R) O X O X X X X Neutral (N) O X X X X X O D Range 1st O O X X X X X .circleincircle. 2nd O O X X X O X .circleincircle. 3rd O O O X X O X 4th X O O O X O X S Range 1st O O X X X X X .circleincircle. 2nd O O X X O O X 3rd O O O X X O X L Range 1st O O X X X X O 2nd O O X X O O X ______________________________________
A hydraulic circuit for actuating these brakes and clutches to obtain the speed ranges shown in Table 1 may be of any well-known construction. Accordingly, a description of the overall hydraulic circuit is deleted except for certain portions thereof discussed below.
Often the exhaust brake apparatus is actuated for a manual downshift from e.g. The S range to first speed in the L range to achieve both exhaust and engine braking for effective braking action. In such case it is necessary to actuate the brake B3. The hydraulic circuit for accomplishing this is shown in FIG. 2.
With reference to FIG. 2, line hydraulic pressure from a regulator valve 1 is admitted into an inlet port 6 of a low-coast valve 5 via an oil line 2, manual valve 3 and oil line 4. Low-coast hydraulic pressure regulated by the low-coast valve 5 enters a 1-2 shift valve 8 via an oil line 7. The valve 8 is shifted to admit the pressure to a chamber for the brake B3.
FIG. 3 shows the relationship between reverse drive torque and engine rotational speed (NE) when the exhaust brake apparatus is turned on and off. The graph clearly shows that reverse drive torque rises with an increase in engine rotational speed. However, as illustrated in FIG. 4, in which output shaft rotational speed (vehicle velocity) is plotted against hydraulic pressure, the hydraulic pressure supplied to the brake B3 is constant with respect to output shaft rotational speed (vehicle velocity) in the prior-art arrangement. The brake B3 is set to a low transfer friction torque volume, which is adapted to a low vehicle velocity (when the exhaust brake is off). In consequence, there is a major deterioration in the durability of the friction members of the brake B3 when engine braking (a downshift from the S range to the L range) is made to take effect at a high vehicle velocity with the exhaust brake on. It is obvious that if a high transfer friction volume is adopted that will accommodate the brake B3 to a high vehicle velocity with the exhaust brake on, the speed-change shock that occurs when a downshift is made will become more pronounced.